1. Field of the Invention
The present invention is directed to processes for converting thermal energy in solar thermal collector/concentrator or co-concentrated solar PV (CPV) systems to electricity.
2. Description of the Related Art
Efficient means to convert energy and produce electricity is important given the increasing demand for electricity. Electricity is generated from thermodynamic engine cycles such as rankine, bryton, steam and gas turbines; hydroelectric and wind turbines as well as solar photovoltaic conversion. Electrochemical routes to generate electricity such as fuel cells, and reverse electrodialysis are also being developed for specific applications.
An enhanced ability to convert thermal energy into electricity offers the potential to improve the efficiency of heat engines and to produce electricity directly from low grade heat. Electricity thus produced would displace energy produced from non-renewable fossil fuels and thereby address concerns associated with fossil fuel use, including resource finiteness, environmental effects (e.g., global warming), and national security.
Myriad processes for conversion of thermal energy to electricity exist. These processes fall into two broad categories: 1) processes that utilize a working fluid to turn a turbine and 2) processes that do not. Turbine processes produce approximately 80% of the electricity consumed globally. Such processes utilize thermal energy to vaporize a compressed working liquid. The vapor is expanded through a turbine to produce electricity and subsequently condensed before recompression and reuse in the cycle.
The efficiency of turbine processes is determined by the efficiency of the underlying Rankine cycle for the working fluid. The efficiency is limited by the temperature difference between the vaporization and condensation steps—the greater the difference the higher the efficiency. The working fluid and operating pressure determine the maximum temperature of the process. This temperature is limited by the mechanical properties and cost of the materials used to construct the boiler and turbine.
As an alternative to the combustion of fossil fuels, solar thermal processes use concentrated solar energy to vaporize the working fluid and are capable of providing high grade thermal energy comparable to that produced from fossil fuels. Geothermal and ocean thermal (utilizing the temperature difference between surface and deep ocean water) sources of thermal energy also may be used. However, the thermal energy is available at lower temperatures, especially with ocean thermal sources, so process efficiency is lower. Waste or low-grade heat is available at low temperatures which inherently limits the efficiency of turbine processes that utilize it. Moreover, choices for working fluids are limited. Consequently, alternative processes for converting thermal energy to electricity are desirable.
Non-turbine processes include those that utilize thermo-electric materials and the Seebeck effect to produce electricity from a temperature difference imposed across bimetallic or p-n junctions. Thermo-electric devices possess the same efficiency limit as Rankine cycle devices.
An emerging area of power generation is the use of the salinity difference between sea water and fresh water runoff from estuaries into the ocean. Pressure retarded osmosis, reverse electrodialysis, and osmotic capacitor processes have been proposed to produce electricity from the mixing of solutions with different osmotic pressures.
Pressure retarded osmosis relies on water transport from a solution of lower osmotic pressure to a solution of higher osmotic pressure. The higher osmotic pressure stream is pressurized and water transport to it produces a flow that can be used to turn a turbine. Membranes that selectively allow water transport relative to salt transport are an essential component of the process.
Reverse electro-dialysis relies on ion transport from higher chemical potential regions to lower chemical potential regions. Membranes that selectively allow transport of either cations or anions are required in the process. Ion transport directly produces an electric current that can be utilized in an external circuit. Reverse electro-dialysis offers the advantage of not requiring a turbine to produce electricity.
The previous work on reverse Electrodialysis was mainly on the seawater and fresh water, or salt solution at ambient temperatures. U.S. Publication No. 2011/0086291A1 discusses mainly the fluid flow distribution pattern, spacing and flow velocity, membrane suitability etc and focuses more on the design geometry of salt solution at ambient temperatures. WO2010/143950A1 discusses fouling and its prevention in reverse electrodialysis by periodic osmotic shock. U.S. Pat. No. 4,171,409 discusses the reverse electrodialysis system for generation of power. Power production from a concentration gradient was first published in 1976 in Science, vol 191, pp 557-9. During the last few years, a lot of effort has been expended by several groups including that by Prof. J. Veerman and his group in Netherlands in reverse electrodialysis. This work specifically focuses on the utilization of process heat produced in the operation of concentrated PV systems and solar energy and waste energy for regeneration of spent feed solutions to its original values so that power production can be produced without the necessity of freshwater and seawater sources. Also, combined utilization of thermal gradient in addition to the salt gradient enhances the ion separation and power production. The increased temperature of operation also reduces the biofouling in the membranes.
In concentrator photovoltaic (CPV) power generation systems, the solar energy is concentrated on solar cells. Voltage drops with rise in temperature of the solar cell and this drop depends on the specific type of semiconductors and its temperature coefficient of voltage. To limit losses due to this voltage drop, it is necessary to cool the solar cell within the acceptable limits. This cooling provides a heat source that could be used as industrial pre-heating. That heat is utilized to produce electricity which in turn increases the overall efficiency of conversion of solar energy. The first is the conventional photovoltaic power generation efficiency which is typically from 15% to 36% depending on the type of the solar cell used in the concentrator. In addition to this, hot reverse electrodialysis produces additional electricity from the coolant waste heat at 45 to 50° C., in which the concentrated salt or ionic liquid solution is heated and circulated in reverse electrodialysis system to produced additional electricity. This approach is a novel way of improving the overall efficiency of the solar energy conversion process.
Similarly, waste heat obtained after expansion of steam in solar steam turbine generator, coolant waste heat produced in solar stirling engine generator, rankine turbine produces enhanced power output through the disclosed hot reverse Electrodialysis in this invention. The current invention paves the way for improving the overall efficiency of the solar thermal power generation process.